1. Technical Field
The present invention relates generally to induction heaters and, in particular, to induction heaters having inverter power supplies.
2. Background Art
Induction heating is a well known method for producing heat in a localized area on a susceptible metal object. Induction heating involves applying a high frequency AC electric signal to a heating loop placed near a specific location on a piece of metal to be heated. The varying current in the loop creates a varying magnetic flux within the metal to be heated. Current is induced in the metal by the magnetic flux and the internal resistance of the metal causes it to heat up in a relatively short period of time. Induction heaters may be used for many different purposes including hardening of metals, brazing, soldering, and other fabrication processes in which heat is a necessary or desirable agent or adjurant.
The prior art is replete with induction heaters, many of which have inverter power supplies. Such inverter power supplies typically develop high frequency signals, generally in the tens of kilohertz range, for application to the work coil. Because there is generally a frequency at which heating is most efficient, some prior art inverter power supplies operate at a frequency selected to optimize heating. Also, because heat intensity is dependent on the magnetic flux created, some prior art induction heaters control the total current provided to the heating coil, thereby controlling the magnetic flux and the heat produced.
One example of the prior art representative of induction heaters having inverters is U.S. Pat. No. 4,092,509, issued May 30, 1978, to Mitchell. Mitchell discloses numerous inverter circuits for powering induction heaters. The circuits are designed to operate in the twenty to fifty kilohertz range, allegedly to maximize induction heating efficiency. To the extent Mitchell discloses controlling the magnitude of the magnetic flux, and therefore controlling the heat created by the induction heater, switches are used to select between one of two inverter circuits. For example, in FIG. 40, switches 404 and 407 are moved to positions 404A and 407A, respectively, or to positions 404B and 407B, respectively, to select between high power output and low power output.
Another known induction heater utilizing an inverter power supply is described in U.S. Pat. No. 3,816,690, issued Jun. 11, 1974, to Mittelmann. Mittelmann describes an induction heater having a variable frequency inverter power supply. The frequency of operation of the inverter is said to be selected to provide the maximum efficiency of energy transfer between the output transformer of the inverter and the inductance element used to heat the workpiece. In order to provide the proper amount of heat to the workpiece, Mittelmann monitors the watt-seconds delivered to the output of the inverter. In response to the measured watt-seconds, Mittelmann selectively turns the inverter on and off. Thus, the average heat delivered by the induction heater is controlled.
Another type of induction heater in which the output is controlled by turning an inverter power supply on and off is disclosed in the U.S. Pat. No. 3,475,674, issued Oct. 28, 1969, to Porterfield, et al. The average output power of the induction heater described by Porterfield varies in accordance with the ratio of the time during which the inverter is off compared to the time during which the inverter is on.
Each of the above methods to control power delivered to an induction heater either is not adjustable in frequency and/or does not control the peak heat delivered by the heater. Accordingly, it is desirable to have an induction heater utilizing an inverter which provides a broad range of frequencies as well as a broad range of peak output heat. The output heat should be controllable independent of frequency and should control the peak as well as the average heat power.